The present invention generally concerns a process for preparing aluminum nitride powder. The present invention more particularly concerns preparing aluminum nitride powder via direct combustion nitridation of aluminum metal.
Aluminum nitride synthesis generally occurs via one of four known processes. One well known process directly reacts aluminum metal with nitrogen (2Al+N.sub.2 .fwdarw.2AlN). A second well known process involves carbothermally reducing and nitriding alumina (Al.sub.2 O.sub.3 +3C+N.sub.2 .fwdarw.2AlN+3CO). A less common process reacts aluminum chloride and ammonia in a vapor phase (AlCl.sub.3 +4NH.sub.3 .fwdarw.AlN+3NH.sub.4 Cl). U.S. Pat. No. 3,128,153 discloses an even less common process wherein aluminum phosphide reacts with ammonia (AlP+NH.sub.3 AlN+1/4P.sub.4 +3/2H.sub.2).
Aluminum nitride part properties depend largely upon purity of the aluminum nitride powder from which they are fabricated. Accordingly, any process which inherently leaves impurities in admixture with aluminum nitride powder should be avoided.
The vapor phase reaction of aluminum chloride and ammonia produces ammonium chloride as a byproduct. Kimura et al., in an article entitled "Synthesis of Fine AlN Powder by Vapor Phase Reaction of AlCl.sub.3 and NH.sub.3 ", Yogyo Kyokai Shi, 96, No. 2, pages 206-210 (1988) teach that ammonium chloride may be removed by high temperature heating. An added complication of the vapor phase synthesis centers upon substantial impurity levels in commercially available aluminum chloride. The aluminum chloride must, therefore, be purified prior to reaction with ammonia.
The reaction of aluminum phosphide with ammonia shares a problem with the vapor phase synthesis. Commercially available aluminum phosphide contains generally unacceptable levels of impurities. In addition, phosphorous is expensive, toxic and highly flammable.
The carbothermal reduction process produces a product which contains residual carbon. The carbon may be oxidized and subsequently removed by a high temperature burnout in air. The burnout also causes some of the aluminum nitride product to oxidize to alumina. The alumina reduces product purity and adversely affects thermal conductivities of articles fabricated from the product.
The direct reaction of aluminum metal to aluminum nitride produces a cleaner product than the other processes for two reasons. First, high purity aluminum is available commercially. Second, the process produces no byproducts.
The direct reaction is exothermic and generates approximately 328 kilojoules per gram-mole of aluminum nitride at 1800 K. Aluminum metal melts at about 933 K. The reaction of aluminum and nitrogen starts at about 1073 K. The reaction, once initiated, is self-propagating if not controlled. An uncontrolled reaction reaches an adiabatic temperature of about 2800 K. Uncontrolled reactions which reach AlN sintering temperatures and remain at these temperatures for extended lengths of time yield sintered aluminum nitride agglomerates. The agglomerates are not readily amenable to further sintering to densities approaching theoretical density via pressureless sintering techniques.
One variation of the direct nitridation process employs plasma reactors to vaporize aluminum metal at temperatures approaching 10,000 K. The vaporized metal then reacts with nitrogen, ammonia, or mixtures of nitrogen and ammonia or nitrogen and hydrogen. The resultant aluminum nitride particles have an average particle size of less than 0.1 micrometer and a surface area of approximately 30 square meters per gram. Baba et al., in "Synthesis and Properties of Ultrafine AlN Powder by RF Plasma", Applied Physics Letters, 54 (23), page 2309 (1989), note that "oxygen contents were found to be roughly proportional to the specific surface area when the powder was exposed in air." They also note that "infrared and nuclear magnetic resonance analysis indicated that the surface of the ultrafine powder was covered with aluminum hydroxide and chemisorbed water." The oxygen reacts with aluminum nitride during sintering to form aluminum oxynitride and reduces thermal conductivity of the resultant sintered product.
Bartlett et al. (U.S. Pat. No. 3,141,737) disclose a process wherein aluminum metal reacts with a cyanamide compound and nitrogen at temperatures between 1373 and 1673 K. for a time sufficient to convert the aluminum to aluminum nitride. They suggest that 1473 K. is an optimum temperature for mixtures heated for periods of 30 to 90 minutes.
Y. Shintaku (U.S. Pat. No. 4,612,045) atomizes molten aluminum into a nitriding atmosphere of heated nitrogen gas. The gas must be at a temperature of 1073 K. or higher. The resultant products reportedly contain an amount of unreacted aluminum metal.
Another variation of the direct nitridation process is known as a floating nitridation process. Atomized aluminum powder is incorporated into a stream of gaseous nitrogen and transferred upward through a heated reaction section. The aluminum powder reacts with the gaseous nitrogen in the reaction section. The resultant product is collected overhead. Typical reaction temperatures range between 1623 and 1823 K. N. Hotta et al., in "Synthesis of AlN by the Nitridation of the Floating Al Particles in N.sub.2 Gas", Yogyo Kyokai-Shi, 95 (2), pages 274-277 (1987), describe a floating nitridation process conducted at temperatures of 1350-1550 degrees Centigrade. They produce very fine aluminum nitride particles having an average size of 0.1 to 0.2 micrometer with reaction times on the order of five seconds. N. Hotta et al., in "Continuous Synthesis and Properties of Fine AlN Powder by Floating Nitridation Technique", Nippon Ceramics Kyokai Gakaujutsu Ronbun-shu, 96 (7), pages 731-35 (1988), report an experimentally determined surface area of 8 square meters per gram and an oxygen content of 1.2 weight percent.
N. Hotta et al., in "Synthesis of AlN by the Nitridation of the Floating Al Particles in N.sub.2 Gas", Yogyo Kyokai-Shi, 95 (2), pages 274-277, report the formation of fibrous aluminum nitride on reactor walls.
H. Yamashita et al. (Japanese Patent Application 01 275472, Sep. 11, 1986) describe a two step floating nitridation process. In step one, aluminum metal in a floating state is reacted with high temperature nitrogen gas for a short period of time at a temperature less than 1473 K. to form a hard aluminum nitride layer only on the surface of the aluminum metal particles. In step two, these intermediate particles are reacted with nitrogen gas at a temperature of 1473 to 2073 K. for a longer period of time. The second step may take place, for example, in a fluidized bed reactor. The resultant aluminum nitride reportedly has an average particle size of one micrometer which equates to a surface area of less than two square meters per gram.